Only prefer Sized candidates, and only if they certainly hold

Author: compiler-errors

compiler/rustc_middle/src/traits/select.rs

@@ -95,8 +95,11 @@ pub type EvaluationCache<'tcx, ENV> = Cache<(ENV, ty::PolyTraitPredicate<'tcx>),
 /// parameter environment.
 #[derive(PartialEq, Eq, Debug, Clone, TypeVisitable)]
 pub enum SelectionCandidate<'tcx> {
-    /// UwU
-    SizedCandidate,
+    /// A built-in implementation for the `Sized` trait. This is preferred
+    /// over all other candidates.
+    SizedCandidate {
+        has_nested: bool,
+    },
 
     /// A builtin implementation for some specific traits, used in cases
     /// where we cannot rely an ordinary library implementations.

compiler/rustc_trait_selection/src/traits/select/candidate_assembly.rs

@@ -1062,8 +1062,10 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidates: &mut SelectionCandidateSet<'tcx>,
     ) {
         match self.sized_conditions(obligation) {
-            BuiltinImplConditions::Where(_) => {
-                candidates.vec.push(SizedCandidate);
+            BuiltinImplConditions::Where(nested) => {
+                candidates
+                    .vec
+                    .push(SizedCandidate { has_nested: !nested.skip_binder().is_empty() });
             }
             BuiltinImplConditions::None => {}
             BuiltinImplConditions::Ambiguous => {

compiler/rustc_trait_selection/src/traits/select/confirmation.rs

@@ -40,8 +40,8 @@ impl<'cx, 'tcx> SelectionContext<'cx, 'tcx> {
         candidate: SelectionCandidate<'tcx>,
     ) -> Result<Selection<'tcx>, SelectionError<'tcx>> {
         let mut impl_src = match candidate {
-            SizedCandidate => {
-                let data = self.confirm_builtin_candidate(obligation, true);
+            SizedCandidate { has_nested } => {
+                let data = self.confirm_builtin_candidate(obligation, has_nested);
                 ImplSource::Builtin(BuiltinImplSource::Misc, data)
             }
 

compiler/rustc_trait_selection/src/traits/select/mod.rs

@@ -1803,25 +1803,19 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
 
         // We prefer `Sized` candidates over everything.
         let mut sized_candidates =
-            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate));
-        if let Some(_sized_candidate) = sized_candidates.next() {
+            candidates.iter().filter(|c| matches!(c.candidate, SizedCandidate { has_nested: _ }));
+        if let Some(sized_candidate) = sized_candidates.next() {
             // There should only ever be a single sized candidate
             // as they would otherwise overlap.
             debug_assert_eq!(sized_candidates.next(), None);
-            return Some(SizedCandidate);
-        }
-
-        // We prefer trivial builtin candidates, i.e. builtin impls without any nested
-        // requirements, over all others. This is a fix for #53123 and prevents winnowing
-        // from accidentally extending the lifetime of a variable.
-        let mut trivial_builtin = candidates
-            .iter()
-            .filter(|c| matches!(c.candidate, BuiltinCandidate { has_nested: false }));
-        if let Some(_trivial) = trivial_builtin.next() {
-            // There should only ever be a single trivial builtin candidate
-            // as they would otherwise overlap.
-            debug_assert_eq!(trivial_builtin.next(), None);
-            return Some(BuiltinCandidate { has_nested: false });
+            // Only prefer the built-in `Sized` candidate if its nested goals are certain.
+            // Otherwise, we may encounter failure later on if inference causes this candidate
+            // to not hold, but a where clause would've applied instead.
+            if sized_candidate.evaluation.must_apply_modulo_regions() {
+                return Some(sized_candidate.candidate.clone());
+            } else {
+                return None;
+            }
         }
 
         // Before we consider where-bounds, we have to deduplicate them here and also
@@ -1950,7 +1944,7 @@ impl<'tcx> SelectionContext<'_, 'tcx> {
             // Don't use impl candidates which overlap with other candidates.
             // This should pretty much only ever happen with malformed impls.
             if candidates.iter().all(|c| match c.candidate {
-                SizedCandidate
+                SizedCandidate { has_nested: _ }
                 | BuiltinCandidate { has_nested: _ }
                 | TransmutabilityCandidate
                 | AutoImplCandidate